Conventional aircraft environmental control systems incorporate an air cycle machine, also referred to as an air cycle cooling machine, for use in cooling and dehumidifying air for supply to the aircraft cabin for occupant comfort. Such air cycle machines may comprise two, three or four wheels disposed at axially spaced intervals along a common shaft, and defining a compressor rotor, a turbine rotor, and one or two additional rotors, for example a fan rotor or an additional turbine rotor or an additional compressor rotor, the turbine or turbines driving both the compressor and the fan. The wheels are supported for rotation about the axis of the shaft on one or more bearing assemblies disposed about the drive shaft. Although the bearing assemblies may be ball bearings or the like, hydrodynamic film bearings, such as gas film foil bearings, are often utilized on state-of-the-art air cycle machines.
Each wheel may comprise only a single rotor, such as, for example, disclosed in commonly assigned U.S. Pat. No. 3,428,242. The three wheel air cycle machine disclosed therein comprises a fan rotor, a turbine rotor and a compressor rotor mounted to a common shaft, with the fan rotor being disposed at one end of the shaft and the turbine and compressor rotors being disposed at the other end of the shaft. The shaft is supported for rotation on a ball bearing assembly disposed intermediate the fan and the turbine and cooled by turbine outlet air. The compressor rotor and the turbine rotor are disposed in back to back relationship on opposite sides of a central plate with the turbine inboard of the compressor. The central plate disposed between the turbine and compressor rotors forms part of the housing encasing the turbine and compressor rotors and defining separate inlet and outlet ducts for the turbine rotor and the compressor rotor. In this arrangement, the central plate is exposed on its outboard side to relatively warmer air being ducted from the compressor rotor and is simultaneously exposed on its inboard side to relatively cooler air being ducted to the turbine rotor.
It is also known in the art for a single wheel to comprise a dual rotor, that is for a single wheel to provide two back-to-back rotors either formed integrally as one piece or integrally mounted together. For example, U.S. Pat. No. 4,312,191, discloses an air cycle machine including a dual rotor wheel mounted on a bearing assembly disposed about an axially extending shaft. This dual rotor wheel comprises a turbine disk and a compressor disk disposed in back-to-back relationship with the compressor disk integrally secured to the turbine disk. The dual rotor wheel is disposed within a housing defining the flow ducts to and from the compressor and turbine rotors and having a central annular plate portion which separates the turbine inlet flow duct from the compressor outlet flow duct. The central plate may be an integral part of the housing or formed by mating two housing segments together to encase the dual rotor wheel. In either case, the central plate is exposed on one side to relatively warmer air being ducted from the compressor rotor, while simultaneously being exposed on its other side to relatively cooler air being ducted to the turbine rotor.
On aircraft powered by turbine engines, the air to be conditioned in the air cycle machine is typically compressed air bled from one or more of the compressor stages of the turbine engine. In conventional systems, this bleed air is passed through the air cycle machine compressor wherein it is further compressed, thence passed through a condensing heat exchanger to cool the compressed air sufficiently to condense moisture therefrom thereby dehumidifying the air before expanding the dehumidified compressed air in the turbine of the air cycle machine to both extract energy from the compressed air so as to drive the shaft and also to cool the expanded turbine exhaust air before it is supplied to the cabin as conditioned cooling air.
The compressed bleed air being supplied to the compressor of the air cycle machine is typically supplied at a temperature of about 105 C. to about 120 C., but raised in temperature during the compression process to a temperature typically in the range about 150 C. to about 175 C. The temperature of the compressed air is thereafter reduced prior to being delivered to the turbine for expansion therein to a temperature typically in the range of about 40 C. to about 50 C. to dehumidify the air, and thence further cooled in the expansion process to a temperature typically less than 5 degrees Celsius above the freezing point of 0 C. Consequently, when the compressor rotor and turbine rotor are disposed in back-to-back relationship with their flow ducts separated by a central plate, the temperature differential across the central plate may range from 80 to 125 degrees Celsius.
Conventionally, the housing, central plate, and rotors of aircraft air cycle machines are made of a light-weight metal, typically aluminum, strong enough to withstand the fluid pressure encountered during operation, but light-weight so as to minimize the impact on fuel consumption during flight. Aluminum, however, has a high thermal conductivity. Thus, an undesireable consequence of this temperature differential across the central plate is heat transfer from the relatively warmer air flow on the compressor side of the central plate, via conduction through the thermally conductive central plate, to the relatively cooler air flow on the turbine side of the central plate, thereby reducing the effective cooling efficiency of the expansion process. Since cooling the air flow is the primary function of the expansion turbine, this undesireable heat transfer resulting from the close proximity of the back-to-back compressor and turbine rotors detracts from the attractiveness of such a back-to-back arrangement, which is generally otherwise desireable as a means of minimizing the overall length, and therefore weight, of the air cycle machine.